Gravel packs are well known techniques for preventing sand from entering a wellbore. A gravel pack is generally formed by placing gravel or sand of a predetermined specific size into the annulus between a casing and a well screen in the wellbore. For effective operation, it is important that the solids in the pack be relatively uniformly distributed with a minimum of voids.
Prior art discloses use of a pulsing fluid flow to stabilize a gravel pack by increasing the density of the gravel pack. However, merely pulsing the flow imparts only a limited amount of energy into the gravel pack.
Another prior art method for stabilizing a gravel pack includes vibrating a drill string and gravel pack apparatus by imparting a sonic frequency vibration at the wellhead. This method is useful if the drill pipe does not have significant contact with the wellbore or casing above the gravel pack. However, in practice, this is rarely the case. Vibrations imparted at the wellhead therefore can be significantly dampened, and vibrations of only a small magnitude may actually arrive at the gravel pack.
Yet another prior art method for producing vibrations for stabilizing a gravel pack uses balls constrained to move within a wash pipe. The balls strike the wash pipe walls which transfer the vibrations to the fluid in the gravel pack annulus. These vibrations in turn are transferred to the gravel pack screen causing the gravel pack solids to vibrate around the screen providing better gravel pack density. This method suffers from low efficiency resulting from the rapid attenuation of vibrations across the wellbore.
Cementing is a routine procedure in oil and gas well drilling operations. During this procedure, a cement-based slurry is pumped into the annulus between the wellbore wall and casing to provide isolation of the reservoir formation from the wellbore as well as to support the casing. This cementing procedure, however, often fails to achieve adequate isolation and there is undesirable communication between the wellbore and the oil-bearing formation.
Inadequate isolation has been attributed to several factors. These include: (1) fluid leaking from the cement into the formation causing a volume reduction when a pressure differential exists across the curing cement; (2) the volume of the slurry contracting up to 6%, during the cement hydration process, allowing micro-annuli to form between the casing and oil-bearing formation; (3) unset cement is very permeable until it develops sufficient strength to prevent fluid influx; and (4) a lowering of the hydrostatic pressure in the annulus that can lead to fluid influx as a result of the development of gel strength in cement slurries. This combination of volume losses due to fluid loss and hydration, loss of hydrostatic pressure because of gel strength and a weak, permeable cement matrix provides an environment for reservoir formation influx to occur.
To overcome these problems, a method of introducing a cement slurry with pulsation was developed. Cement pulsation is the application of low intensity pressure pulses to the annulus after a primary cement job thereby delaying gel strength development in the cement slurry. This method, however, cannot prevent voids from arising during deposition of the cement slurry and water outflow from the slurry into the formation.
Vibrations have also been used to enhance the packing of the solid particulates in cement slurries during the cementing operations. In one instance, vibration has been achieved when a bluff object impacts the walls of the cement slurry flow conduit. The energy of impact depends on the flow velocity and that may be too low to provide the necessary improved packing effect.
In view of the problems discussed above, a more effective method for generating vibrations for use in gravel pack and cementing operations in oil and gas well construction is required. The method should generate strong vibrations with little attenuation and be introducible with as little damping as possible. The method should supply the necessary amount of energy to all intervals of the gravel pack or casing for the required amount of time. The method should also supply the necessary amount of energy for solids stimulation and be effectively independent of flow velocity or other factors.